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SPRI Review 2003: Polar Physical Science

Polar Physical Science

Unsteady ocean melting of Antarctic ice

Floating ice shelves fringe the majority of the Antarctic coastline, forming a natural barrier between grounded ice and the surrounding oceans. Ice melting at this boundary accounts for around half of the water mass loss from the entire Antarctic continent, and so fluctuations in ocean temperatures are of considerable interest. Today, the greatest Antarctic melt rates occur in regions where warm waters have access to large quantities of ice. In the Amundsen Sea sector, massive outlet glaciers discharge into Pine Island Bay, where warm circumpolar deep water (CDW) has infiltrated for some time. This water — around 4°C above the freezing point — melts more than 40 m of ice at the base of glaciers each year, in a region of western Antarctica that is thinning rapidly due to a disturbance in glacier flow dynamics. The presence of CDW is therefore a key focus for our investigations as to the source of the Amundsen Sea mass imbalance. Elsewhere in Antarctica, oceans had been considered to be broadly in a state of balance with the ice sheet. This view is now changing, as new satellite measurements have revealed a previously undetected source of instability at the Antarctic Peninsula. Surface lowering rates show that the Larsen Ice Shelf has progressively thinned during the past decade — a period in which large sections have rapidly disintegrated. The basal ice thinning provides a new mechanism for ice shelf collapse, a process that was previously attributed to atmospheric warming, and suggests that more sectors of Antarctica may be exposed to warm ocean currents than has been considered thus far. Two papers on this work have appeared recently in the international journal Science.

Andy Shepherd

Mass fluctuations of small ice bodies

The contribution of polar ice sheets to twentieth-century global sea-level rise has been effectively quantified through combinations of satellite radar surveys. Although smaller ice bodies cannot provide any sizable additional contribution, their timescale for response to changes in climate is far shorter than that of the great ice sheets, and so observations of fluctuations in their mass and dynamics may be used as validation datasets for climate-change scenarios. The forthcoming CryoSat interferometric radar altimeter provides a new opportunity to quantify the mass balance of small ice bodies, a task that is beyond the capacity of Envisat instrumentation and, potentially, the rescheduled IceSat orbit. Investigations of CryoSat swath-mode retrievals over regions of steep terrain in Svalbard will determine the efficacy of elevation measurements from small ice bodies. These data will be of great value for surveys of mass balance, and for constraining interferometric measurements of ice motion.

Andy Shepherd

Topographic controls on glacier mass-balance sensitivity

A distributed surface energy balance has been used to determine hourly totals of the surface energy balance components, and hence melt, over entire glacier surfaces throughout a melt season, with a spatial resolution of 20 m. From these results, the spatially averaged mass-balance/elevation profile for the glacier can be calculated. For Haut Glacier d'Arolla, Switzerland, a cubic relationship with elevation gives the best fit to the calculated mass balance curve. The shape of this profile varies with the imposed change in meteorological conditions, however, becoming increasingly 'S' shaped for warmer or less snowy conditions. These mass-balance profile changes are due to the complex interplay between albedo variations due to different snow depths over the glacier surface (and eventual removal of the snow cover), the variations in solar energy receipts caused by slope and aspect variation over the glacier, and the changing patterns of shading by the surrounding topography. The changes in mass-balance profile lead to maximum calculated mass-balance sensitivity to imposed change occurring at intermediate elevations on the glacier; the calculated equilibrium line altitude occurs at the upper end of this zone, resulting in very large calculated changes in the ELA for different climatic conditions.

Neil Arnold

Submarine landforms and the reconstruction of fast-flowing ice streams within a large Quaternary ice sheet

Morphological interpretation of regional and detailed bathymetric data sets on the 2500 km-long Norwegian shelf from the North Sea (57°N) to Svalbard (80°N) has revealed a dynamic ice flow pattern along the western, 2500 km-long margin of the Scandinavian and Barents/Svalbard ice sheets. About 20 cross-shelf troughs with megascale glacial lineations (elongate ridges and grooves oriented parallel to trough long axes) are interpreted as pathways for fast-flowing paleo-ice streams. Studies of large-scale margin morphology and seismic profiles have identified large submarine fans at the mouths of major cross-shelf troughs. On shallower banks, less dynamic ice probably existed. The two largest former ice streams were the Norwegian Channel Ice Stream and Bear Island Trough Ice Stream, each 150-200 km wide at the mouth. The onset of fast flow is generally close to the outer coast, at the border zone between crystalline rocks and softer sedimentary rocks. Transverse submarine ridges on various scales, often parallel to the shelf edge, reflect either the maximum ice-sheet position or the recessional pattern of the ice sheet. Lateral ice stream moraines several tens of kilometres long have also been mapped along the sides of several cross-shelf troughs, identifying the border zone between fast ice flow and stagnant or slow-flowing ice on intervening banks. This work has been undertaken in collaboration with Dag Ottesen and Lief Rise of the Norwegian Geological Survey.

Julian Dowdeswell

Continental slope morphology and sedimentary processes at the mouth of an Antarctic palaeo-ice stream

Continental-slope and shelf-edge morphology off Marguerite Bay, western Antarctic Peninsula, was investigated using swath-bathymetric data and parametric sub-bottom profiler records, together with sediment cores. Marguerite Bay has a well-defined cross-shelf trough, and a relatively steep continental slope. The slope beyond the trough mouth is convex in longitudinal profile, whereas to the north and south it is concave and reaches a maximum of 12°. There are no deep canyons cutting into the prograding outer shelf and slope. Instead, a series of gullies runs down the upper slope, reaching depths of >200 m south of the trough mouth but <120 m deep beyond the trough. The mid- and lower slope appears to be relatively smooth, and downslope sediment transfer is probably by small-scale slides, slumps, and debris flows. The continental rise contains dendritic channels related to turbidity currents, and sediment drifts produced by southwest flowing bottom currents from the fine-grained component of the turbidity currents. Elongate sedimentary bedforms indicate that a fast-flowing ice stream occupied the trough under full glacial conditions, and transferred deforming subglacial till rapidly to the shelf edge. By contrast, on either side of the trough mouth, ice is inferred to have been slower moving and probably cold-based, delivering little sediment to the upper slope. The steepness of the continental slope results in rapid down-slope sediment transfer by debris flows, slumps, and turbidity currents, and accounts for the lack of a well-developed trough-mouth fan, which is typical of many lower-gradient glacier-influenced margins. The work was collaborative with Carol Pudsey of the British Antarctic Survey and funded by a grant from the NERC Antarctic Funding Initiative.

Julian Dowdeswell and Colm Ó Cofaigh

Snow hydrology on temperate and polythermal glaciers

This work is concerned with improving the snow-hydrology component of a distributed glacier-hydrology model, and the application of this model to both isothermal and cold snowpacks. Data to develop and test the model have been collected at Haut Glacier d'Arolla, Switzerland, and Midre Lovénbreen, Svalbard. There are two components to the model. The first uses a distributed version of the 1D (vertical) snow energy and mass balance model SNTHERM to calculate patterns of melting and vertical routing of water across a glacier during the summer. Output from this model component is tested against measurements of surface albedo, surface ablation, snowpack temperatures, and lysimeter measurements of water outflow from the base of the unsaturated snowpack. The second component uses the 3-D groundwater model MODFLOW to route the water laterally through the saturated layer at the base of the snowpack. This component of the model is tested against measurements of water-table depth within the saturated layer. The coupled model can be used to calculate spatial and temporal patterns of surface melt and water storage in the unsaturated and saturated parts of the snowpack, and temporal patterns of runoff in the proglacial stream. This work is being undertaken in collaboration with Neil Arnold and our former PhD student, Andy Fox (Durham).

Ian Willis

Sediment deformation beneath Icelandic glaciers

A study employing macro- and micro-sedimentological techniques is being conducted on sediments exposed on recently deglaciated fore-fields at nine surging and non-surging glaciers in Iceland. We are interested in using these techniques to identify the relative importance of sliding vs sediment deformation beneath the glaciers, the styles (ductile vs brittle) and pervasiveness (depth) of deformation, and whether these vary in space and time. Macro-scale evidence includes particle-size distribution and individual clast shape, angularity, and fabric. Micro-scale evidence involves the identification of rotational structures, fold structures, and unistrial plasmic fabric (evidence for ductile deformation), and alignment of grains, shear lines, and crushed grains (evidence for brittle deformation) in thin sections viewed under a micro-scope. Provisional results from the surge-type Brúarjökull imply that a lower till unit underwent limited brittle shear at low pore-water pressures during an early surge advance, ice-bed de-coupling was common at the height of the surge due to high pore-water pressures, and the deposition and deformation of an upper till unit occurred at moderate pore-water pressures during the waning stages of the surge and subsequently. This work is being undertaken in collaboration with Colm Ó Cofaigh and our current PhD student, Anna Nelson.

Ian Willis

Self-similarity in glacier surface characteristics

Catchment-wide information on glacier snow-cover depth, surface albedo, and surface roughness are important input data for distributed models of glacier energy balance. In this study, we investigate the small-scale (mm to 100 m) spatial variability in these properties, with a view to better simulating this variability in such models. Data were collected on Midre Lovénbreen, a 6 km2 valley glacier in northwest Svalbard. The spatial variability of all three properties was found to be self-similar (fractal) over the range of scales under investigation. Snow depth and albedo exhibit a correlation length within which measurements were spatially autocorrelated. Late winter and summer properties of snow depth were different, with smaller depths in summer due to melt, and shorter correlation lengths. Similar correlation lengths for snow depth and surface albedo may suggest that snow-depth variation is an important control on the small-scale spatial variability of glacier surface albedo, one of the main controls on glacier melt rates.

Neil Arnold and Gareth Rees

Impact of fractal snow-cover distribution on glacier melt rates

Snow-depth distribution over a glacier is one of the key controls on glacier mass balance. Accumulation of snow obviously forms the 'positive' side of the mass-balance equation, but it also plays a key role in determining the amount of ablation on a glacier, and hence the 'negative' side of the mass-balance equation, as it is one of the primary controls on glacier albedo and surface roughness. Previous work in SPRI has shown that snow-cover distributions are fractal over a wide range of spatial scales; this enables such distributions to be simulated mathematically, and used in a distributed surface energy balance model. Initial results from such models show that fractal snow-depth distributions increase the total amount of melt over glaciers, as areas of shallower snow have a greater impact on melt than areas of deeper snow. As the melt season progresses, such fractal distributions also give rise to the 'patchy' snow cover typically observed in the field. Following the successful acquisition of remotely sensed data in the summer of 2003, we are expanding the spatial scale of these simulations, and exploring the impact of different fractal models for snow-depth distributions on calculated melt.

Neil Arnold and Gareth Rees

Reindeer herding in the circum-Arctic region

Herding of reindeer forms an important component of the social and economic position of indigenous peoples across the circum-Arctic region. The pressures on this way of life are great, including both natural (climatic variability, predation, and disease) and anthropogenic (climate forcing, pollution, changes in land access, changes in the socio-political setting) factors. In turn, reindeer form an important part of sub-Arctic ecosystems, playing a major role in controlling the distribution and dynamics of high-latitude vegetation. The Institute is engaged in a multidisciplinary investigation — involving social anthropology, landscape analysis, and other methods — of the relative importance of these factors. Geographical Information Systems (GIS) and remote-sensing techniques are being used to examine reindeer herding areas in the Barents Sea region (northern Norway, Sweden, Finland, and European Russia) and to model reindeer habitat, with the goal of improving our understanding of potential climate change effects. Project analyses are being conducted on two scales: as part of the international research collaboration BALANCE, a general assessment of the Barents Sea region on a coarse scale (20 km grid); and a detailed analysis of reindeer habitat in a specific study area within the Nenets Autonomous Okrug (NAO) in Russia at a fine scale (100 m grid). Fieldwork in the NAO was begun in summer 2003. Satellite data are currently being used in conjunction with vegetation data from the field to produce a vegetation map of the study area in the NAO. Field, geophysical, and other relevant data will later be processed and analysed within the multi-layered GIS to create the desired habitat models and the modelled changes on vegetation composition and distribution caused by potential climate change eventually incorporated. A broadly similar study is being conducted on reindeer herding within the Komi Republic, Russian Federation, with the principal focus on identifying changes in land use as a result of collectivisation and subsequent political changes. An interdisciplinary approach has been adopted that involves botanical mapping (using satellite remote sensing), ethnobiology (knowledge of reindeer herding technology), and archival work in Russian administration centres.

Gareth Rees, Mark Dwyer, Fiona Danks, and Piers Vitebsky